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BIG PHYSICS, BIG QUESTIONS –

Technology: Britain dithers on digital Channel 5 . . .

By BARRY FOX

The Independent Television Commission has passed the buck on whether
Channel 5, the proposed new TV service, should be digital or analogue. The
decision will now be made by Peter Brooke, the minister at the Department
of National Heritage.

Every day’s delay makes an analogue version less attractive as the consortium
of European manufacturers and broadcasters involved in their own ‘Digital
Video Broadcasting’ project head towards inaugurating Europe’s first digital
TV services in late summer 1995. The DVB forum now numbers more than 100
members from 12 countries, and intends to start digital TV services from
the Astra satellite and German cable TV stations. Meanwhile Japan has admitted
that the rest of the world is shifting to digital transmission for high-definition
TV (see story this page).

Brooke’s department will now take advice from the Department of Trade
and Industry’s Radiocommunications Agency and wait for the ITC to finish
further research on the use of fallow frequencies before making a decision.
All this, admits the department, will take several months.

The BBC has publicly said that the analogue frequencies reserved for
Channel 5 should now be used for digital TV, and the ITC itself is ‘very
doubtful’ that technical details contained in the proposals from companies
that want to start an analogue service would manage to avoid interference
to thousands of video recorders and TVs tuned to BBC2.

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But the ITC has received too many similar proposals to be able to ignore
them. ‘We are obliged under the Broadcasting Act 1990 to do all we can to
establish a fifth terrestrial channel, and there is a strong case for re-advertising’,
says James Conway of the ITC. ‘Only the government can release us from that
. We now need a clear steer on what the government wants’.

The two frequencies reserved for Channel 5 are known as channels 35
and 37, and are already used by many millions of viewers to connect video
recorders, satellite tuners and video games to TV sets. If Channel 5 broadcasts
on those frequencies it will create widespread interference which the franchised
broadcaster must cure by retuning or modifying domestic equipment.

The ITC first auctioned the licence in 1992, but refused to award it
to the single applicant which was backed by Thames Televison. Last year,
the ITC published a consultation document and received 70 submissions, some
of which argued that frequencies other than channels 35 and 37 were available
in some cities.

The recently formed alliance of MAI, Pearson and Time-Warner claims
to have ‘found’ spare frequencies which will ‘significantly’ reduce the
risk of interference. MAI cites UHF channel 48 in Glasgow, and channel 28
in London. But there are good technical reasons why these are not used.

The tuning oscillator in a TV set is always 39.5 megahertz above the
channel being watched. So the set generates spurious interference signals
at a frequency 39.5MHz above the tuned channel. British TV channels are
8MHz apart, so the oscillator’s interference is only 0.5MHz away from the
programme which is five channels higher than the one being watched. This
causes a herring-bone pattern of interference on the screen.

Thus every fifth channel is unused for broadcasting. In London, channel
28 would suffer interference from sets tuned to ITV on channel 23. In Central
Scotland, channel 48 is five channels up from ITV. Any sets tuned to ITV
risk interfering with nearby sets tuned to the ‘newly found’ frequencies
suggested for Channel Five.

BBC engineers warn that such methods also put the reception of BBC programmes
at risk. If Channel Five were to use ‘newly found’ channel 28, any set tuned
to it could interfere with nearby sets tuned to BBC2, on channel
33.